Marty McFly: Wait a minute, Doc. Ah... Are you telling me you built a time machine... out of a DeLorean?
Dr. Emmett Brown: The way I see it, if you're gonna build a time machine into a car, why not do it with some style?
~ Back to the Future...
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// A covered bridge is a bridge, often single-lane, with enclosed sides and a roof. They have typically been wooden, although some newer ones are concrete or metal with glass sides. Especially associated with the nineteenth century, covered bridges often serve as prominent local landmarks and have long attracted the attentions of historical preservationists.
Contents [hide]
1 Covered bridges in Europe
2 Covered bridges in North America
3 Covered bridges in Asia
4 Modern covered bridges
5 Covered bridges in fiction
6 Gallery of Covered Bridges
7 See also
8 External links
[edit] Covered bridges in Europe
The Western tradition of covered bridges originated in Central Europe. The European covered bridge consisted of a roof and siding that protected a wooden truss structure, thus contributing to bridge longevity.
Surviving or reconstructed European covered bridges include:
Bridge over the Saane/Sarine river, near Fribourg, Switzerland (picture)
Kapellbrücke, near Lucerne, Switzerland — 650 foot long, built 1333
The Covered Bridge in Lovech, Bulgaria — built 1874
Logic Lane covered bridge in Oxford, England — built 1904
Famous stone covered bridges include the Rialto Bridge in Venice, Italy which is one of only three over the Canal Grande and a popular tourist attraction.
The Bridges of Sighs in Venice, Cambridge and Oxford are also covered bridges.
Green River bridge in Guilford, Vermont
[edit] Covered bridges in North America
Such bridges are found in rural areas throughout the United States and Canada, but are often threatened by arsonists, vandals, and flooding. In the United States, Pennsylvania has more covered bridges (more than 200) than any other state, many of which can be seen in Chester County, Pennsylvania and Lancaster County, Pennsylvania. They are also common in places such as Elizabethton, Tennessee, Lane County, Oregon, Madison County, Iowa, and Parke County, Indiana. Parts of Ohio, Michigan, Kentucky, Maryland, Minnesota, Virginia, West Virginia and the New England states also have surviving covered bridges.
There are various structural designs used for covered bridges, such as the Burr arch truss.
Opened on July 4, 1901, the 1,282 foot (390 meter) Hartland Bridge, crossing the Saint John River at Hartland, New Brunswick, Canada, is currently the longest covered bridge in the world. It is a Canadian National Historic Site. In 1900, New Brunswick had an estimated 400 covered bridges, and Quebec more than 1000, while Ontario had only had 5. As of 2006, there were 94 covered bridges still standing in Quebec, 65 in New Brunswick and one in Ontario.
A much longer covered bridge (5,960 feet) between Columbia and Wrightsville, Pennsylvania once spanned the mile-wide Susquehanna River, making it the longest and most versatile covered bridge in the world during its existence. It featured railroad tracks, a towpath for canal boats crossing the river between two canals on either bank, and a carriage / wagon / pedestrian road. The popular tollbridge was burned June 28, 1863, by Union militia during the American Civil War to prevent its usage by the Confederate Army of Northern Virginia during the Gettysburg Campaign. A replacement wooden covered bridge was destroyed by a windstorm a few years later. It was rebuilt as an open-air steel bridge.
Covered bridges are generally considered old-fashioned, and appeal to tourists, but the purpose is twofold: (1) covered bridges appear similar to barns and it is easier to transport cattle across them without startling them, and (2) to build a structure for weather protection over the working part of the bridge. A bridge built entirely out of wood, without any protective coating, may last 10 to 15 years. Builders discovered that if the bridge's underpinnings were protected with a roof, the bridge could stand for 70, or even 80 years. The existing covered bridges have been renovated using concrete footings and steel trusses to hold additional weight and to replace the original support timbers.Some covered bridges, such as the one in Newton Falls, Ohio and Elizabethton, Tennessee, also feature an integrated covered walkway.
Dong Minority Bridge, Chenyang, Guizhou, China. (2004-08-16)
[edit] Covered bridges in Asia
There are many covered bridges, called Wind and Rain Bridges in the Chinese province of Guizhou. These were traditionally built by the Dong minority people.
Taishun County, in the Zhejiang province, has more than 900 covered bridges, many of them hundreds of years old, as well as a covered bridge museum.[1]
[edit] Modern covered bridges
Plank-lattice truss interior structure of Green River bridge in Guilford, VermontModern covered bridges are usually for pedestrians, for example to walk from one part of an office building to another part, to cross railway tracks at a station, or in a shopping center on an elevated level, crossing a road. See also skyway.
Glass-walled covered bridges are rather common at American airports, and some of those bridges can be found at John F. Kennedy Airport in New York City.
Also, some highway bridges, such as the George Washington Bridge, have lower decks for additional capacity, and those decks, while generally open on the sides, can be enclosed with plastic from time to time during construction, thus rendering the lower decks as partially covered bridges.
[edit] Covered bridges in fiction
Baumgardener's Covered Bridge, employing the Burr Arch TrussNorth American covered bridges received much recognition as a result of the success of the novel, The Bridges of Madison County written by Robert James Waller and made into a Hollywood motion picture starring Meryl Streep and Clint Eastwood.
The fictional rural town portrayed in the 1988 film Beetlejuice features a covered bridge. It provides the early scene in which the protagonists (played by Alec Baldwin and Geena Davis) are killed when their car crashes through the wall of the bridge and plunges into the river below.
A covered bridge is featured in the 1999 film Sleepy Hollow, in a suspense-filled scene depicting an encounter between main character Ichabod Crane (played by Johnny Depp) and the main villain, The Headless Horseman (played by Christopher Walken).
In the early 20th century, covered bridges were sometimes nicknamed "kissing bridges", as the cover allowed seclusion for couples to kiss each other.
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// [edit] Other theories
[edit] The possibility of paradoxes
The Novikov self-consistency principle and recent calculations by Kip S. Thorne[citation needed] indicate that simple masses passing through time travel wormholes could never engender paradoxes—there are no initial conditions that lead to paradox once time travel is introduced. If his results can be generalised, they would suggest, curiously, that none of the supposed paradoxes formulated in time travel stories can actually be formulated at a precise physical level: that is, that any situation you can set up in a time travel story turns out to permit many consistent solutions. The circumstances might, however, turn out to be almost unbelievably strange.[citation needed]
Parallel universes might provide a way out of paradoxes. Everett's many-worlds interpretation of quantum mechanics suggests that all possible quantum events can occur in mutually exclusive histories.[citation needed] These alternate, or parallel, histories would form a branching tree symbolizing all possible outcomes of any interaction. If all possibilities exist, any paradoxes could be explained by having the paradoxical events happening in a different universe. This concept is most often used in science-fiction, but some physicists such as David Deutsch have suggested that if time travel is possible and the many-worlds interpretation is correct, then a time traveler should indeed end up in a different history than the one he started from. [16]
Daniel Greenberger and Karl Svozil proposed that quantum theory gives a model for time travel without paradoxes. [17] In quantum theory observation causes possible states to 'collapse' into one measured state; hence, the past observed from the present is deterministic (it has only one possible state), but the present observed from the past has many possible states until our actions cause it to collapse into one state. Our actions will then be seen to have been inevitable.
The science fiction writer Larry Niven made his own suggestion about paradox avoidance in his essay The Theory and Practice of Time Travel. He argued that as long as time travel exists, history will change, and will only become static when a timeline is reached in which no time travel exists and thus no further changes can be made (unless it is on a course that is repetitive or will never become static, but there is no evidence of this). Assuming there is only a single dimension of time, the timeline we perceive must be the one that exists after all changes (if any) are made, and thus we will never perceive the invention of time travel, since it will have already destabilised itself out of the timeline by the 'time' we would have reached it.[citation needed] However, few if any physicists or philosophers have taken seriously the possibility of "changing" the past except in the case of multiple universes, and in fact many have argued that this idea is logically incoherent (see this discussion between two philosophers, for example), so this idea is not usually seen outside of science fiction.
[edit] Using quantum entanglement
Quantum-mechanical phenomena such as quantum teleportation, the EPR paradox, or quantum entanglement might appear to create a mechanism that allows for faster-than-light (FTL) communication or time travel, and in fact some interpretations of quantum mechanics such as the Bohm interpretation presume that some information is being exchanged between particles instantaneously in order to maintain correlations between particles.[citation needed] This effect was referred to as "spooky action at a distance" by Einstein.
Nevertheless, the rules of quantum mechanics curiously appear to prevent an outsider from using these methods to actually transmit useful information, and therefore do not appear to allow for time travel or FTL communication. The fact that these quantum phenomena apparently do not allow FTL/time travel is often overlooked in popular press coverage of quantum teleportation experiments. How the rules of quantum mechanics work to preserve causality is an active area of research.
[edit] Time travel in fiction
// Abstraction. It is still possible today to grant Thorndike's point that identical elements underlie the phenomenon of transfer. However, research suggests that a more complex picture of how identical elements figure in the process of transfer. An identity that mediates transfer can sit at a very high level of abstraction. Phenomena such as the branching of arteries and that of electrical power networks can evince the same deep principle (the need to deliver something to a region point by point) with great differences in what constitutes a conduit (arteries versus wires) and the something being carried (blood versus electricity). Such a degree of abstraction helps to account for far transfer, because highly abstract identical elements can appear in very different contexts.
Transfer by affordances. Writing from the perspective of situated cognition, Greeno et al. (in press) argue that transfer need not depend on mental representations that apply to the learning and target situations. Rather, during initial learning, the learner may acquire an action schema responsive to the affordances ─ the action opportunities ─ of the learning situation. If the potential transfer situation presents similar affordances and the person recognizes them, the person may apply the same or a somewhat adapted action schema there. External or internal representations may or may not figure in the initial learning or the resulting action schema.
High road and low road transfer. Salomon and Perkins (1989, Perkins and Salomon 1987) synthesized findings concerned with transfer by recognizing two distinct but related mechanisms, the ``low road'' and the ``high road.'' Low road transfer happens when stimulus conditions in the transfer context are sufficiently similar to those in a prior context of learning to trigger well-developed semi-automatic responses. In keeping with the view of Greeno et al. (in press), these responses need not be mediated by external or mental representations. A relatively reflexive process, low road transfer figures most often in near transfer. For example, when a person moving a household rents a small truck for the first time, the person finds that the familiar steering wheel, shift, and other features evoke useful car-driving responses. Driving the truck is almost automatic, although in small ways a different task.
High road transfer, in contrast, depends on mindful abstraction from the context of learning or application and a deliberate search for connections: What is the general pattern? What is needed? What principles might apply? What is known that might help? Such transfer is not in general reflexive. It demands time for exploration and the investment of mental effort. It can easily accomplish far transfer, bridging between contexts as remote as arteries and electrical networks or strategies of chess play and politics. For instance, a person new to politics but familiar with chess might carry over the chess principle of control of the center, pondering what it would mean to control the political center.
In a particular episode of transfer, the two roads can work together ─ some connections can occur reflexively while others are sought out. But in principle the two mechanisms are distinct.
This framework matches well a number of the points made earlier. It acknowledges that sometimes transfer is stimulus driven, occurring more or less automatically as a function of much and diverse practice (the low road). On the other hand, sometimes transfer involves high levels of abstraction and challenges of initial detection of possible connections (the high road). The framework makes room for identical elements in Thorndike's original sense ─ identities that the organism simply responds to (the low road) ─ but insists on the importance of identities discovered and exploited by mindful exploration (the high road).
This analysis along with the views and findings of Luria, Scribner and Cole, Greeno, and others emphasizes that the conditions for transfer are stringent. Reflexive (low road) transfer requires well-automatized patterns of response that are thus easily triggered by similar stimulus conditions ─ and it requires stimulus conditions enough like prior contexts of learning to act as triggers. Many learning situations offer practice only for a narrow range of examples and not enough practice to achieve significant automaticity, providing a poor basis for reflexive transfer. Mindful (high road) transfer requires active abstraction and exploration of possible connections. Many learning situations do not encourage such mental investments, although people more inclined to mindfulness or metacognition are by definition more likely to make them.
6. Teaching for Transfer
These points about mechanism clarify why transfer does not occur as often as would be wished. They also provide guidelines for establishing conditions of learning that encourage transfer.
In many situations, transfer will indeed take care of itself ─ situations where the conditions of reflexive transfer are met more or less automatically. For example, instruction in reading normally involves extensive practice with diverse materials to the point of considerable automaticity. Moreover, when students face occasions of reading outside of school ─ newspapers, books, assembly directions, and so on ─ the printed page provides a blatant stimulus to evoke reading skills.
In contrast, in many other contexts of learning, the conditions for transfer are less propitious. For example, social studies are normally taught with the expectation that history will provide a lens through which to see contemporary events. Yet the instruction all too commonly does not include any actual practice in looking at current events with a historical perspective. Nor are learners encouraged to reflect upon the eras they are studying and extract general widely applicable conclusions or even questions. In other words, the conventions of instruction work against both automatic (low road) and mindful (high road) transfer.
In response to such dilemmas, one can define two broad instructional strategies to foster transfer: hugging and bridging (Perkins and Salomon 1988). Hugging exploits reflexive transfer. It recommends that instruction directly engage the learners in approximations to the performances desired. For example, a teacher might give students trial exams rather than just talking about exam technique, or a job counselor might engage students in simulated interviews rather than just talking about good interview conduct. The learning experience thus ``hugs'' the target performance, maximizing likelihood later of automatic low road transfer.
Bridging exploits the high road to transfer. In bridging, the instruction encourages the making of abstractions, searches for possible connections, mindfulness, and metacognition. For example, a teacher might ask students to devise an exam strategy based on their past experience, a job counselor might ask students to reflect on their strong points and weak points and make a plan to highlight the former and downplay the latter in an interview. The instruction thus would emphasize deliberate abstract analysis and planning. Of course, in the cases of exam technique and job interview, the teachers might do both. Instruction that incorporates the realistic experiential character of hugging and the thoughtful analytic character of bridging seems most likely to yield rich transfer.
In summary, a superficial look at how research on transfer casts its vote is discouraging. The preponderance of studies suggest that transfer comes hard. However, a closer examination of the conditions under which transfer does and does not occur and the mechanisms at work presents a more positive picture. Education can achieve abundant transfer if it is designed to do so.
D. N. Perkins
Harvard Graduate School of Education
Cambridge, Massachusetts, U.S.A
G. Salomon
University of Haifa
Haifa, Israel
University of Arizona
Tucson, Arizona, U.S.A.
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